1. Field of the Invention
The present invention relates to an image recording apparatus for scanning a printing head in a main scanning direction while moving an object to be printed in a subscanning direction, thereby forming an image.
2. Related Background Art
A conventional image recording apparatus for scanning a printing head in a main scanning direction while moving an object to be printed in a subscanning direction, thereby forming an image, has a printing portion, as shown in FIG. 11A. A platen 2 has suction holes 3 for chucking a sheet 1 as an object to be recorded. A suction pipe 4 is connected to the platen 2 to evacuate the platen 2 so as to chuck the sheet 1 through the holes 3. A printing head 5 is mounted on a head carriage 6 to oppose the platen 2. A sheet holder 7 holds the sheet 1 between the platen 2 and the printing head 5. The main scanning direction is a direction perpendicular to the drawing surface, and the subscanning direction is a vertical direction.
The plurality of suction holes 3 ar distributed in the surface of the platen 2 within a print width of the printing head 5. The suction pipe 4 is connected to a suction pump (not shown). Upon operation of the suction pump, the inner space of the platen 2 is set to be a negative pressure, so that the sheet 1 can be chucked on the platen 2 through the suction holes 3. Therefore, floating of the sheet 1 on the platen 2 can be prevented. Chucking of the sheet through suction holes is known in the prior art (e.g, Japanese Patent Laid-Open (Kokai) No. 61-280964).
FIG. 11B shows a sheet suction position of the second to last line when a remaining subscanning length of the image area is divided by a print width of the printing head 5. Referring to FIG. 11B, a hatched portion of the sheet 1 represents a printed portion. If an area in which the suction holes 3 are equidistantly formed within the print width along the subscanning direction is defined as X, and an area without the holes 3 and not overlapping the sheet holder 7 is defined as Y, condition X=Y is established.
A length Z of a portion (the lower end portion of the sheet 1) of the sheet 1 held by the sheet holder 7 is equal to a blank portion in which an image is not formed and represented by Z. In this manner, in a state wherein the lower end portion of the sheet 1 is kept held by the sheet holder 7, an upper portion of part of the sheet 1 which is located on the platen 2 is chucked by the suction holes 3, while its lower portion is held by the sheet holder 7. Therefore, floating of the sheet 1 on the platen 2 can be prevented.
FIG. 11C shows a printing position at which the last line is printed on the sheet 1. Although the blank portion Z is not chucked by the suction holes 3, floating of the blank portion Z almost never occurs because its width is about 2 mm. When printing of the last line and the second to last line is performed in the pattern described above, the sheet 1 having a floating lower end is not caught by the printing head 5 during its main-scanning movement.
However, the subscanning remaining length of the sheet is not necessarily divided by the head print width. A portion between the last printing line and the lower end of the sheet 1 may have a length smaller than the head print width X. In this case, in a state wherein the second to last line is to be printed, as shown in FIG. 12, the portion having the length smaller than the head print width X is removed from the sheet holder 7 and is not chucked by the suction holes 3. In this manner, the lower end of the sheet 1 is curled to cause trouble such as jamming.
It may be possible to shift the sheet holder 7 to the upper portion. However, it is actually difficult to shift it because of the shape of the printing head 5. It may also be possible to form the suction holes 3 throughout the entire area of the platen 2. However, power of the suction pump must then be increased, and the structure is complicated, and power consumption is undesirably increased. The portion having the length smaller than the head print width X may be assigned to the uppermost line. However, this proposal cannot cope with a manual paper feed mode in which a subscanning remaining length is discriminated near the last printing line.